1. Field of the invention
The invention relates to a method and a device for producing tubes of a heat exchanger, in particular a preheater in a power plant, the tubes being rolled in hydraulically in a tube plate and a rounded tube inlet being formed.
2. Discussion of Background
In heat exchangers used in power plants, the tubes through which water flows are fixed in tube plates by means of rolling-in techniques. For this purpose, a tube is firstly placed into a bore of a tube plate, and the mandrel of the rolling machine is then inserted into the tube and widens the tube in a plastic fashion such that said tube adheres to the inner wall of the bore. As a rule, the rolling-in fixes and seals the tube by means of the adhesive forces between the bore wall and the tube outer wall. The tightness of the connection between the tube and tube plate is improved by welding, the welded seam serving only as a sealing seam and contributing nothing to the adhesive forces. Mechanical and hydraulic rolling-in techniques are presently distinguished. Mechanical rolling-in typically extends in this case over a depth of 20 to 30 mm, which is mostly smaller than that of the tube plate. Rolling-in over the entire tube plate depth is certainly possible using this method, but proves to be time-consuming and, with some tube materials, difficult. The hydraulic rolling method, as carried out, for example, by the Teco tube expanders company in Wuppertal, Germany, uses hydraulic widening of the tube instead of mechanical widening. This permits widening without any problem independently of the tube material, and the fastening of the tube over the entire tube plate depth, the resulting strength of the connection being increased. Furthermore, hydraulic widening over the entire tube plate depth permits the use of thinner and more cost-effective tube plates, since such a widening means that the tube wall thickness can be regarded as a bearing part in the tube plate.
In preheaters used in power plants, in particular in high-pressure preheaters, damage due to erosion at the tube inlet occurs relatively frequently. Such damage is rectified successfully by so-called inserts. These inserts, as described in British Patent 1,141,239, for example, are trumpet-shaped structures which are fixed at the tube inlet by tube widening and project slightly above the surface of the tube plate. The trumpet shape of the inlet piece of the tube is also known by the term "bellmouth inlet". The round shape of the insert favors the inflow by reducing the radial velocity gradients at the transition from the water chamber to the tube, as well as reducing the shear forces at the tube inner wall. The resistance values of the inflow in the case of such a tube inlet are discussed, for example, in I. E. Idelchick, Handbook of Hydraulic Resistance, page 126, 2nd Edition, Springer Verlag (1986). A low resistance value has the effect, on the one hand, of reducing the inflow pressure loss and, on the other hand, of sparing the oxide layer of the tube and generally reducing the erosion and corrosion of the tube. In addition, the tube inlet pieces are produced in each case from erosion- and corrosion-resistant materials. By projecting beyond the tube plate surface by a multiple of the tube outside diameter, the inserts further yield the advantage that a "water cushion" which strongly reduces the erosion of the tube plate is formed in the region of the inflow field. However, it is disadvantageous with such inserts that, because of the rolling-in in the region of the rolling-in site, a site of faltering flow which increases the risk of cleavage cracks and corrosion forming on the latter is formed at the seam between the insert and tube. Finally, the subsequent fastening of inserts occasions additional costs.